The present disclosure relates to image reading apparatuses for use in copying machines, facsimile machines, multifunctional peripherals incorporating the entirety or some of functions of these machines, image scanners, or the like, and image forming apparatuses including the image reading apparatuses.
An image forming apparatus forms an electrostatic latent image on a surface of a photosensitive member by causing an image reading apparatus to read an image, on a document sheet, to be copied, and applying laser light to the photosensitive member that is an image carrier, based on document sheet image data having been read. The image reading apparatus applies light from a linearly-arranged light source to the document sheet placed on a document sheet table, and receives light reflected by the document sheet, via a mirror or the like, by a photoelectric converting portion. The photoelectric converting portion converts the received light to an electrical signal. Thus, the document sheet image data is obtained. As a light source that lights a document sheet, a xenon lamp, a light-emitting diode, or the like is used. In recent years, reduction in size and power-saving are required for lighting devices, and image reading apparatuses that use light-emitting diodes as light sources are known.
As lighting devices using light-emitting diodes, lighting devices that use a mode in which multiple light-emitting diodes are disposed along a longitudinal direction (the primary scanning direction) of the lighting device, and lighting devices that use a mode in which light of one light-emitting diode disposed at an end portion in the longitudinal direction is diffused by a light guide member over the entirety of a region in the longitudinal direction, are used. In the mode in which one light-emitting diode is disposed at an end portion in the longitudinal direction, cost is low, and mounting operation is relatively easily performed. However, in this mode, an amount of light with which a document sheet is lit tends to become insufficient, and an amount of light from the lighting device needs to be increased. In order to increase an amount of light from the lighting device, a light-emitting diode that has an enhanced brightness is necessary. Therefore, an electric current to be applied to the light-emitting diode needs to be increased. In this case, increase of an electric current to be applied causes increase in heat value from the light-emitting diode. If heat value from the light-emitting diode increases, in a case where document sheet images are read continuously over a long time period, the temperature of the light-emitting diode rises, and the light-emitting diode is thus likely to be damaged or an amount of emitted light is likely to be reduced. Further, heat is transmitted to optical components, such as a mirror or a lens, near the light-emitting diode, an image sensor of a photoelectric converting portion, or the like. Therefore, a performance for an image formation on the photoelectric converting portion is likely to be degraded.
An associated technique for dissipating heat from a light-emitting diode and reducing rising of temperature in the light-emitting diode, is known. For example, in a publicly known image reading apparatus, a light-emitting diode is mounted onto a substrate, and the substrate having the light-emitting diode mounted thereon is joined to a heat dissipating member made of aluminium, through a heat transmission member. A plurality of heat-dissipating fins are disposed on an outer surface of the heat dissipating member, thereby dissipating heat generated in the light-emitting diode.
In general, in order to enhance a heat dissipating efficiency of the heat dissipating member, a plurality of heat dissipating fins are disposed in the heat dissipating member, or the size of the heat dissipating member itself is increased. Therefore, the size of the lighting device having the heat dissipating member provided therein is increased. Further, in the image reading apparatus, in order to perform reading by scanning a document sheet, a carriage having the lighting device is moved in a secondary scanning direction, and when the reading the document sheet is completed, the carriage is returned to an initial position. In order to stop the carriage at the initial position, the initial position of the carriage is detected by a position detecting portion having a photointerrupter or the like, and a driving motor for the carriage is stopped based on the detection result. In order to detect the initial position of the carriage by means of the position detecting portion, a detection member that moves in the secondary scanning direction according to the carriage reciprocating, is necessary. When the detection member is provided on the outer circumferential portion of the carriage as well as the heat dissipating member is provided in the lighting device as described above, the size of the image reading apparatus is further increased.